Washington DC: Former security contractor Edward Snowden was able to obtain secret documents revealing a massive US spying effort from the National Security Agency's internal website, US officials said according to a report on Wednesday.

The classified documents leaked by Snowden were posted internally, and Snowden's job allowed him to single-handedly make digital copies without his supervisors' knowledge, government officials told National Public Radio.

They did not tell NPR how Snowden took copied files out of the office, citing an ongoing investigation.

"We have an extremely good idea of exactly what data he got access to and how exactly he got access to it," NSA's chief technology officer, Lonny Anderson, told NPR.

Anderson said the agency has taken steps to limit employees' options for storing data since the NSA surveillance programs were revealed.

"One thing we have done post-media leaks is lock those down hard, so those are [now] all in two-person control areas," he told NPR's "Morning Edition" program.

Snowden disclosed secret NSA programs involving the collection of telephone and email data to media outlets, including The Guardian and The Washington Post, which began publishing details in June.

He is wanted on US espionage charges and is living in temporary asylum in Russia.

The NSA disclosures have raised questions about US surveillance efforts and privacy as well as private contractors' clearance procedures and access to sensitive data.

But changes to data-sharing could also have national security implications given the push to share more intelligence among agencies after the September 11, 2001, attacks. Information sharing also arose as an issue in the Boston marathon bombing in April.

Anderson said other changes include limiting access to sensitive documents by "tagging" them with identifiers that will also allow supervisors to see who is viewing what data and what those individuals do with it.

The NSA's internal website still exists but it would not be possible for anyone now to make such copies without risk of detection, he added.